Hieron went to live with John and Theo Hieronymous when he was a newborn due to the fact that his mother had trouble raising him. He was officially adopted by the couple when he was eight years old.[1]

After being kicked off the wrestling team at Hofstra University, Hieron began dealing drugs before being charged with a felony.[2] He turned to boxing to let his aggressions out on the bags. Finding this to be a good stress-reliever, and with the encouragement of some friends who trained in MMA, Hieron combined his wrestling skills and burgeoning boxing prowess into a professional MMA career.

In March 2006, Hieron won the Lockdown in Paradise in Lahaina, Hawaii, and in June 2006 won his IFL debut at the IFL Team Championships in Atlantic City, New Jersey.

Following the demise of the International Fight League due to financial concerns, Hieron signed with Affliction in August 2008.[3] However, that promotion soon folded as well, so Hieron signed with Strikeforce.

Hieron's first Strikeforce fight was set to be against fellow former Zuffa competitor Nick Diaz at Strikeforce: Carano vs. Cyborg for the inaugural Strikeforce welterweight title. However, Diaz missed a pre-fight drug test mandated by the California State Athletic Commission and was denied a license to compete. Jesse Taylor was named to replace Diaz and the fight was changed to a non-title bout.[4] Hieron won by unanimous decision. He had his heart set on a title shot against Nick Diaz, but has stated that he would be comfortable fighting any top level opponent, as long as the fight, and name is helping him move forward in his career.

Hieron faced Joe Riggs on January 30, 2010 at Strikeforce: Miami.[5] Hieron won the fight via unanimous decision. After the fight had finished, Hieron's contract with Strikeforce was completed, prompting Hieron to immediately call for demands should he sign a new one. The demands included a shot at champion Nick Diaz and guaranteed television spots.

Hieron and Explosion Entertainment were unable to come to terms, so Hieron instead ended up signing to fight on Bellator Fighting Championships in the promotion's Season 4 welterweight tournament. In the opening quarterfinal round Hieron won via controversial stoppage when referee Josh Rosenthal stopped the bout when Lapsley did not respond to a hand check. Lapsley contested he wasn't out and this appeared to be the case, but given the position, it is unlikely that he would have survived the round. Hieron stated in a post-fight interview that Lapsley was gurgling and he felt him going out.[6] Hieron won a controversial unanimous decision victory over Brent Weedman at Bellator 40 to move on to the tournament finals. At Bellator 43 Hieron defeated Rick Hawn via split decision to win the Season 4 Welterweight Tournament. With this victory Hieron earned a title shot against Bellator Welterweight Champion Ben Askren. Askren defeated Hieron via controversial split decision.

Hieron was expected to face Jake Ellenberger in a rematch on September 1, 2012 at UFC 151, replacing an injured Josh Koscheck.[7] However, after the UFC 151 event was cancelled, Ellenberger/Hieron took place on October 5, 2012 at UFC on FX 5.[8] Hieron lost via unanimous decision.

Hieron was expected to face Erick Silva on February 2, 2013 at UFC 156.[9] However, Silva pulled out of the bout citing an injury and was replaced by promotional newcomer Tyron Woodley.[10] He lost the fight KO (punches) in round 1 and was subsequently released from the promotion.[11]

On March 7, 2014, Hieron announced his retirement from MMA competition.[12]